1. Field of the Invention
The present invention relates to an information transmitting apparatus arranged to transmit information by using flash light emission.
2. Description of Related Art
Typical examples of known information transmitting apparatuses arranged to transmit optical information include remote controllers of TV sets, video tape recorders and the like. These remote controllers are arranged to use an infrared-emitting diode (IRED) as a light emitting device. The infrared-emitting diode has a quick responsivity and is capable of accurately transmitting information at a high speed. However, a shortcoming of the infrared-emitting diode lies in that the light emission output thereof is small and can reach only a short distance of several meters.
The reachable distance can be made longer by using a light emitting device having an intense light output. In the past, some apparatus was developed to use a flash light emission tube having a large amount of light output for the light emitting device. For example, a light release device disclosed in Japanese Laid-Open Utility Model Application No. SHO 55-99529 is arranged to control a shutter release action by emitting flash light from a signal transmitting side at intervals as shown in FIG. 6, and by receiving the flash light on the side of a photo-taking apparatus such as a camera or the like.
Further, a camera system disclosed in Japanese Laid-Open Patent Application No. HEI 4-343336 also uses a flash light emission tube as a light emitting device. This camera system is arranged, as shown in FIG. 7, to cause the flash light emission tube of a built-in flash light emission device of the camera to emit two control pulses at a predetermined interval and, upon lapse of a predetermined period of time after emission of the two control pulses, to emit one light emission start pulse for a slave flash device (a wireless flash device) in synchronism with the full open of the shutter.
Compared with the conventional arrangement for using a light emitting diode, the arrangement for using a flash light emission tube as a light emitting device is capable of emitting a light signal (the control pulses and the light emission start pulse shown in FIG. 7) which has light emission energy per pulse several hundred or several thousand times as much as the light emission energy of the conventional arrangement and can reach a very far distance.
However, in a case where a flash light emission tube is used as the light emitting device, the following problem arises. When the flash light emission tube is in a cool state, i.e., in a case where a light pulse is emitted after the lapse of a long interval time from the previous light pulse emission, a time lag of scores of microseconds takes place before the actual start of light emission from the flash light emission tube even with a high trigger voltage applied to cause the flash light emission tube to start light emission. In a case where the flash device is caused to continuously emit light at time intervals of one hundred microseconds or thereabout, on the other hand, the ions of gas such as Xe (xenon) gas sealed in the flash light emission tube still remain in a sufficient quantity within the flash light emission tube. When a trigger signal is applied to the flash light emission tube under such a condition, light emission immediately begins. In other words, the time lag before actual light emission after application of a trigger signal is long in the case of a long interval between the first light emission or previous light emission and the next light emission, and is short while light emission is continuously made. Under such a condition, the intervals of light emission pulses become uneven to make accurate communication impossible.
FIGS. 8(A) to 8(D) show in a timing chart, by way of example, how accurate information communication is caused to become impossible by the delays, or time lags, of light emission from a flash light emission tube. FIG. 8(A) shows a synchronizing clock signal which is a reference signal to be used for conducting optical communication. An optical information signal is sent out at intervals which coincide with the synchronizing clock signal. FIG. 8(B) shows the information signal to be sent in synchronism with the synchronizing clock signal. For example, a signal of "10001111" is sent. In FIG. 8(B), reference symbol START denotes a signal which is added in front of information signal data transmitted to give information of the start of signal transmission to a signal receiving device. FIG. 8(C) shows light pulses actually emitted from the flash light emission tube when a trigger signal is applied to the flash light emission tube in synchronism with the information signal shown in FIG. 8(B). As shown in FIG. 8(C), the first light pulse P1 synchronized with the START signal and the intermediate light pulse P3, each of which is emitted after the lapse of a long interval from the preceding light emission, lag and delay to a great extent from their corresponding parts of the information signal shown in FIG. 8(B). The light pulse P2 and the light pulses P4 to P6, each of which is emitted continuously from the preceding light emission, do not show much lagging. Therefore, the intervals of light pulse emission become uneven.
On the other hand, as shown in FIG. 8(D), since the signal receiving device is arranged to make checks for the presence or absence of the light pulses at intervals of a fixed period of time and only during a short period of time, after receipt of the light pulse P1 emitted in synchronism with the START signal, the information signal transmitted as "10001111" from the signal transmitting side would be received as "00001000" because only the light pulse P3 which is emitted after about the same extent of lag (delay) as the light pulse P1 can be recognized. Under such a condition, it is hardly possible to accurately conduct communication.